Wet process phosphoric acid, such as that obtained from phosphate rock deposits in central Florida, contains fluorine in amounts sufficient to make it unsuitable for use in animal feed additives, human food additives, or as a raw material for the preparation of phosphate compounds used in detergent formulations. In addition, the presence of fluorine in the wet process phosphoric acid causes post precipitation of complex fluorides which hamper production of "merchant" acid grades of phosphoric acid. Environmental considerations may also require the removal of fluorine from phosphoric acid.
Various methods of defluorination have been proposed. Precipitation with salts of insoluble fluorine-containing compounds is not very useful, because substantial quantities of phosphoric anhydride are also precipitated. This represents a significant economic loss of phosphate values. Vacuum concentration has been tried, but does not remove enough fluorine to warrant the expense.
A more widely practiced process for removing fluorine from wet process phosphoric comprises the addition of silica, such as diatomaceous earth, to concentrated impure acid (54% P.sub.2 O.sub.5). Diatomaceous earth (tripolite, kieselguhr) is formed from the siliceous shells of diatoms and is found in the beds of ancient seas in places such as the Great Basin of Nevada. The material is also used for filtering, as an absorbent, an insulation material, and as an abrasive in soaps. In addition to these competing uses, the cost of diatomaceous earth is increased by the transportation costs to the phosphate rock mine sites.
New government regulations have forced the construction of collectors for the fumes issuing from submerged arc furnaces used to produce ferroalloys such as ferrosilicon or other silicon alloys. The fume is collected as a dust material from the furnace exhaust gases and trapped in bag filters.
Until now, there were very few methods of disposal of the collected fume dust. It could be dumped in a land fill, discharged into sludge ponds, sold as a raw material for making refractory bricks, gunning mixes and prefabricated chimneys. Silica fume dust can also be used as a mold wash when mixed with water in some ferroalloy casting procedures, and the dust may be used as an insulation to cover molten alloys in ladles.
The above uses are not sufficient in volume to keep up with the large quantities of ferrosilicon dust which is collected. The subject invention provides a new use for this waste product, and considerably lowers the cost of defluorinating phosphoric acid.